How does QOS work?

QOS essentially means giving preference to one traffic type over another. For example, sending voice traffic before email traffic. When a broadband network becomes busy (e.g. during evening “peak” hours), voice traffic QOS ensures call quality, even though download/upload speeds for other traffic types (e.g. email) may decrease.

To provide QOS two things are needed. The first is a means to identify traffic types, e.g. a marker for voice traffic, another for email, another for video and so on. Then a method is needed to give priority to one traffic type over another.

Traffic identification may be done by inserting a “priority” field at the front of each data “packet”. The NBN is based on Ethernet technology, where a 3 bit “Priority Code Point” field at the start of each Ethernet packet allows the QOS “class” to be identified.

For example, a Priority Code Point value of 5 (the three bits set to “101”) might be assigned to all Ethernet voice call packets. This identifies the voice packets as highest priority. A lower priority value (e.g. 4) might be assigned to all video conferencing packets. Best effort packets may then be assigned a Priority Code Point value of 0.

Once the various traffic types have been identified, using the Priority Code Point field, then preferential treatment can be applied. Essentially this means a separate “queue” for each traffic class at the various network nodes. Traffic (e.g. Ethernet packets) in the highest priority queues are sent first, the lowest priority queue goes last, the others in between.

There are many other details, but this is essentially how QOS is applied.

How does the NBN provide QOS?

The NBN identifies 4 QOS categories, or classes. These are:

  • Traffic Class 1 (TC_1): meant for voice or other services with strict delay requirements
  • Traffic Class 2 (TC_2): meant for interactive video services
  • Traffic Class 3 (TC_3): enterprise data services, e.g. with guaranteed bandwidth
  • Traffic Class 4 (TC_4): best effort

Traffic Class 1 has the highest priority, Traffic Class 4 (the default class) has the lowest priority.

The standard NBNCo “Access VC”, which will underpin most subscriber broadband services, provides 150 kbps of TC_1 capacity. This is for the NBN supported voice service, which replaces the previous fixed line voice service. For the default Access VC, all additional subscriber traffic is designated Traffic Class 4 (TC_4), i.e. best effort with no guarantees.

Several methods may be used to indicate or mark traffic belonging to the four NBN Traffic Classes. A key method will be the Ethernet Priority Code Point (PCP) field outlined above. Here Ethernet frames (or packets) are assigned PCP values as follows:

TC_1 gets a PCP value of 5
TC_2 gets a PCP value of 4
TC_2 gets a PCP value of either 3 or 2
TC_4 gets a PCP value of 1

Ethernet packets with higher PCP values are assigned higher priorities within the respective NBN nodes. Hence Traffic Class 1 (TC_1), with a PCP value of 5, gets the highest priority.

In particular, the various NBN traffic classes apply within the “Network Terminal Unit” or NTU installed within subscriber premises (the box replacing the current broadband gateway). This NTU recognises the 4 NBN traffic classes described above, and essentially sorts incoming traffic (from the subscriber) into 4 queues, one for each traffic class.

These queues determine the order in which traffic from the 4 respective classes is sent into the NBN, hence providing the required QOS capability. Similar mechanisms are used within the NBN to enforce the four traffic classes for incoming traffic to the subscriber.

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